1. Field of the Invention
The present invention relates to a radiation imaging device that converts radiation into visible light and obtains a radiographic image.
2. Description Related to the Prior Art
As a type of medical imaging system using radiation such as X-rays, an X-ray image capturing system is known. The X-ray image capturing system is constituted of an X-ray generating apparatus for applying the X-rays to a patient's body part to be examined, and an X-ray image capturing apparatus for taking an X-ray image of the body part. The X-ray image capturing apparatus has a radiation imaging device that captures a radiographic image as an electric image. A type of radiation imaging device using a FPD (flat panel detector) is recently in practical use. The FPD has a detection surface formed of a matrix of pixels each of which accumulates signal charge by an amount corresponding to an amount of the X-rays incident thereon. In the FPD, the signal charge is accumulated in the detection surface on a pixel-by-pixel basis, and the accumulated signal charge i.e. the detected X-ray image is outputted in the form of digital image data.
There are known two types of FPD, that is, a direct conversion type and an indirect conversion type. In the direct conversion type FPD, the X-rays are directly converted into the signal charge by a conversion layer made of amorphous selenium or the like. In the indirect conversion type FPD, the X-rays are temporarily converted into visible light, and the visible light is converted into the signal charge. The indirect conversion type FPD is constituted of a scintillator for converting the X-rays into the visible light, a detection panel opposed to the scintillator, and an electric control circuit. The detection panel has the detection surface in which a photoelectric conversion layer for producing the signal charge by photoelectric conversion is formed on a pixel-by-pixel basis on an electrical insulating substrate such as a glass substrate. Thus, the detection panel converts the visible light from the scintillator into the signal charge, and accumulates the signal charge.
The FPD is used in the form of an FPD cassette (electronic cassette), in which the FPD is contained in a flat portable housing, in addition to a state of being set in a floor-mounted imaging support for taking a radiograph of the patient in a standing or lying position. The electronic cassette is formed into the same size as a conventional radiographic cassette, including a film cassette using an X-ray photographic film and an IP cassette using an IP (imaging plate). For this reason, the electronic cassette can be used in the conventional X-ray image capturing system designed for the conventional radiographic cassette, and this is especially convenient when taking a radiograph of a body part (for example, elbow or knee) that is hard to take with the floor-mounted system. Furthermore, the electronic cassette can be used for bedside radiography, which is carried out for the patient who cannot move from his/her bed.
There are three essential conditions required of the housing of the electronic cassette. Firstly, the housing must be lightweight enough to enable portability. Secondly, the housing must be highly X-ray transparent, because a front surface of the housing becomes an X-ray irradiation surface through which the X-rays enter the FPD. Thirdly, the X-ray irradiation surface of the housing must have rigidity enough to endure a load imposed by the patient's body part, because when the electronic cassette detached from the imaging support is used on a bed or a table, the body part to be examined is put on the X-ray irradiation surface of the housing.
To satisfy the essential conditions required of the housing of the electronic cassette, according to Japanese Patent Laid-Open Publication No. 2009-156936, a top plate disposed in the X-ray irradiation surface of the housing is made of CFRP (carbon-fiber reinforced plastic), for example, being a lightweight, rigid, and highly X-ray transparent carbon material. The general CFRP is formed of a lamination of prepregs. Each prepreg is made of carbon fibers oriented in one direction and impregnated with resin. In the CFRP, the prepregs are laminated such that the orientation of the carbon fibers differs between any of the two prepregs overlapping each other.
With the aim of making uniform temperature distribution of the detection panel, according to another conventional X-ray imaging device, an anisotropic heat transfer carbon sheet is disposed on an X-ray incident side of the detection panel (refer to U.S. Pat. No. 7,714,295 corresponding to Japanese Patent Laid-Open Publication No. 2009-085639, for example).
The detection panel of the FPD is more sensitive to temperature change than the X-ray photographic film or the IP. Thus, temperature variations occurring in the detection surface of the detection panel easily manifest themselves in the form of image density variations. In the electronic cassette, a projection plane of the top plate of the housing is overlaid on the detection surface of the detection panel. The top plate and the detection panel are disposed near to each other or make tightly contact with each other to slim the housing. For this reason, heat of the top plate is easily transferred to the detection panel. If the temperature variations occur in the top plate by a partial temperature increase, the temperature variations occur in the detection panel. When the electric charge accumulated in each pixel of the detection panel is read out by a readout circuit through plural signal lines, the heat sometimes interferes with the operation of the readout circuit, and causes noise in the X-ray image.
The indirect type FPD adopts either an ISS (irradiation side sampling) method or a PSS (penetration side sampling) method between which the layout of the scintillator and the detection panel is different. In the ISS method, the top plate, the detection panel, and the scintillator are disposed in this order from an X-ray irradiation side, such that an X-ray incident surface of the scintillator is opposed to the detection surface of the detection panel. In the PSS method, on the other hand, the top plate, the scintillator, and the detection panel are disposed in this order, and the detection panel detects the visible light that has reached a surface opposite to the X-ray incident surface of the scintillator. The ISS method is superior in detection efficiency, because the visible light produced in the X-ray incident surface of the scintillator is received immediately by the detection panel without attenuation. In the ISS method, however, the detection panel is nearer to the top plate, and hence is more susceptible to heat from the top plate than in the PSS method.
For making the detection panel thin or flexible, it is studied to change the substrate of the detection panel from the conventional glass substrate to a resin substrate, or to omit the substrate itself. In this case, the heat of the top plate is transferred to the pixels more easily, so further affects the performance of the detection panel.
The Japanese Patent Laid-Open Publication No. 2009-156936 does not disclose the temperature variations caused by body heat of the patient transferred from the top plate to the detection panel, and measures against the temperature variations. The U.S. Pat. No. 7,714,295 aims to reduce the temperature variations caused by heat produced by the detection panel itself, and never discloses prevention of the temperature variations caused by the body heat of the patient transferred from the top plate.